This invention relates to circuitry for controlling the output level of high intensity discharge lamps and, more particularly, to circuitry having load-side control using a controlled impedance coupled between a transformer primary winding and the high intensity discharge lamp. The controlled impedance, and a circuit rendering the controlled impedance conductive for predetermined periods of time, control energization of the lamp and therefore the lighting output.
High intensity discharge (HID) lamps are used in conjunction with a ballast and include capacitors and complex magnetic circuits to limit lamp current. Of the various types of ballasts available for use, high intensity discharge lamp systems most frequently are used in conjunction with lead type ballasts. Lead type ballasts use negative magnetic properties to their advantage. Magnetization current is used to improve power factor correction and core saturation is used to regulate lamp current. An additional advantage is gained in that both the magnetization current and core saturation factors result in minimizing the size and cost of the ballast.
While high intensity discharge lamps using mercury or metal halide may be grouped together as conventional discharge fixtures, high pressure sodium (HPS) lamps are effectively in a separate group requiring larger ballasts due to lamp voltage increasing with hours of operation. Furthermore, HPS lamps require a relatively large voltage to initiate the arc. The ballast in the case of an HPS lamp circuit must, therefore, provide power regulation, as well as the starting voltage.
Numerous techniques have been proposed for controlling the output illumination level of gas discharge lamps. U.S. Pat. No. 4,197,485 provides lamp dimming in response to selected illumination levels or varying secondary sources such as natural sunlight. U.S. Pat. Nos. 4,207,497 and 4,207,498 convert line frequency to a higher frequency in an effort to increase the efficiency of the unit.
The assignee of the present patent application has three pending U.S. patent applications on lighting controls for fluorescent lamps. The first two of three applications, Ser. Nos. 286,770 filed July 27, 1981 and 309,460 filed Oct. 7, 1981, concern control circuits functioning with standard magnetic ballasts to control the output illumination level of gas discharge lamps. In order to accommodate a multitude of magnetic ballasts, the control circuits each include a circulating inductor coupled in parallel with a controlled impedance such that current is provided to the lamps during a portion of the AC signal when the controlled impedance would be nonconductive. The third application, Ser. No. 338,340 filed Jan. 11, 1982, provides similarly functioning control circuitry in conjunction with an integral ballast and is intended for replacement of existing ballasts or new manufacture.